まる、さんかく、しかく (circle, triangle, square) are the shapes that I have taught my toddler so far. For older children, click here for a more complete list of all the shapes and their names.
I think this is a cute art activity for preschoolers to help them learn the names of shapes. First, cut out various shapes from colored paper. Then get creative and make animals, buildings, etc! I found the idea at マトリョーシカな日々 blog.
Kids-points.com has dot-to-dot worksheets where your toddler/preschooler can practice tracing shapes, here.
Isn’t there a song called ”まる、さんかく、しかく”? I seem to remember learning it when I was little but I can’t find it online. Do you know?
Just found another cute video:
Hiragana Mama 
まる、さんかく、しかく の歌、CDあるよ〜。
たしか子供向けの番組(ポンキッキだと思う）の中の歌でした。